1. Field of the Invention
The present invention generally relates to semiconductor devices and, more particularly, to a semiconductor device having a substrate provided with an through hole to which a protruding electrode such as a solder ball is provided.
Recently, in association with reduction in the size of semiconductor devices and increase in density of circuits in semiconductor devices, the fine pitch BGA (Ball Grid Array) structure has become widely used for semiconductor devices. A semiconductor device having the fine pitch BGA structure comprises a substrate whose front surface is provided with a semiconductor chip and a resin package molding the semiconductor chip. The back surface of the substrate is provided with solder balls as external connection electrodes.
In order to further reduce the size of the semiconductor device having the fine pitch BGA structure and further increase the density of circuits in the semiconductor device having the fine pitch BGA, the pitch of the solder balls must be further reduced. However, since a high reliability is required for semiconductor devices, a predetermined level of reliability must be maintained even when the pitch of the solder balls is reduced.
2. Description of the Related Art
FIGS. 1 and 2 show parts of semiconductor devices 1A and 1B having a typical fine-pitch ball grid array (FBGA) structure, respectively.
The semiconductor device 1A shown in FIG. 1 is generally referred to as an over molding type BGA. The semiconductor device 1A comprises a substrate 2, a semiconductor chip 3, a plastic package 8 and solder balls 10 (only one shown in the figure).
The substrate 2 is made of a plastic film. The semiconductor chip 3 is mounted on the substrate 2 via an adhesive 4. Through holes 7 are formed at predetermined positions of the substrate 2. Only one through hole 7 is shown in FIG. 1. The opening of the through hole 7 on the surface on which the semiconductor chip 3 is mounted is closed by an electrode film 5 which is formed by copper (Cu) or gold (Au) plating.
A via part 9 is formed inside the through hole 7. The solder ball 10 is bonded to the via part 9. Accordingly, the solder ball 10 is electrically connected to the electrode film 5 via the via part 9. The solder ball 10 protrudes from the surface of the substrate 2 so as to serve as an external connection terminal.
In the over molding type semiconductor device 1A shown in FIG. 1, an electrode of the semiconductor chip 3 is electrically connected to the electrode 5 by a wire 6. The plastic package 8 is formed by, for example, transfer molding so as to protect the semiconductor chip 3, the electrode film 5 and the wire 6.
The semiconductor device 1B shown in FIG. 2 is generally referred to as a flip-chip type FBGA. In FIG. 2, parts that are the same as the parts shown in FIG. 1 are given the same reference numerals, and descriptions thereof will be omitted. Stud bumps 11 are formed on the semiconductor chip 3 (only one stud bump 11 is shown in FIG. 2). The stud bumps 11 may be solder bumps. The semiconductor chip 3 is flip chip bonded to the electrode films 5.
Each of the semiconductor devices 1A and 1B has solder balls 10 as external connection electrodes. Accordingly, the manufacturing process of each of the semiconductor devices 1A and 1B includes a ball mounting process to mount the solder balls 10 onto the substrate 2.
FIGS. 3 through 5 show a method for mounting the solder ball 10 to the substrate 2. It should be noted that FIGS. 3 through 5 show a method for manufacturing the semiconductor device 1A shown in FIG. 1.
In the ball mounting method shown in FIG. 3, an appropriate amount of flux 12 (or solder paste) is previously applied to the solder ball 10, and the solder ball 10 is inserted into the through hole 7 formed in the substrate 2. FIG. 4 shows a state in which the solder ball 10 is inserted into the through hole 7.
Conventionally, a pitch (ball pitch) between the adjacent solder balls is as large as about 0.8 mm. Thus, the diameter L1 of the through hole 7 can be as large as 0.30 mm to 0.40 mm. The diameter R of the solder ball 10 is 0.40 mm to 0.50 mm. Accordingly, when the solder ball 10 is applied to the through hole 7, the entirety of the solder ball 10 is accommodated in the through hole 7 as shown in FIG. 4, or most of the solder ball 10 is accommodated in the through hole 7.
After the solder ball 10 is accommodated in the through hole 7, a solder reflow process (heating process) is performed. Conventionally, since the entirety or most of the solder ball 10 is accommodated in the through hole 7, the melted solder ball 10 positively fills the through hole 7 and is bonded to the electrode film 5. Additionally, excess solder forms the solder ball 10 on the substrate 2 due to a surface tension of the melted solder ball 10. As a result, the semiconductor device 1A shown in FIG. 1 is formed.
On the other hand, in the ball mounting method shown in FIG. 5, an appropriate amount of solder paste 13 is applied to the interior of the through hole 7 according to a screen printing method. As mentioned above, the diameter L1 of the conventional through hole 7 is sufficiently large, therefore an appropriate amount of the solder paste 13 can be easily applied inside the through hole 7. It should be noted that the solder paste 13 is a mixture of the flux made of an organic material and a solder powder.
Thereafter, the solder ball 10 is applied to the through hole 7 in which the solder paste 13 is applied, and a solder reflow process is performed. Thereby, the organic component contained in the solder paste 13 scatters, and the solder powder is melted, which fills the through hole 7. Additionally, the solder ball 10 is also melted and brought into contact with the melted solder in the through hole 7, which results in formation of the semiconductor device 1A shown in FIG. 1.
As mentioned above, the number of terminals provided in a single semiconductor device has been increased due to increase in the density of the semiconductor chips. Additionally, there is a demand for the semiconductor devices to be further reduced in size since the electronic equipment in which the semiconductor devices are incorporated is required to be smaller.
Accordingly, the ball pitch of the solder balls provided in the semiconductor device has become as small as 0.5 mm. In order to achieve the ball pitch of 0.5 mm, the diameter L1 of each through hole must be in the range of 0.20 mm to 0.25 mm. Additionally, the diameter of each solder ball must be about 0.3 mm.
If the ball mounting method mentioned with reference to FIGS. 3 and 4 is used to form the solder balls of the semiconductor device having the above-mentioned small ball pitch, each solder ball cannot be appropriately inserted into the respective through hole 7 since the diameter of the through hole 7 is much smaller than the diameter of the solder ball 10. Accordingly, a large distance remains between the solder ball 10 and the electrode film 5. Thus, even if the solder reflow process is performed, there is a problem in that the solder ball 10 is not electrically connected to the electrode film 5.
FIGS. 6A and 6B show an example in which the ball mounting method mentioned with reference to FIG. 5 is applied to the substrate 2 having a through hole 14 having a diameter of 0.20 mm. When the diameter of the through hole 14 is as small as 0.20 mm to 0.25 mm as shown in FIG. 6A, an appropriate amount of the solder paste 13 cannot be filled into the through hole 14 by using a screen printing method. That is, the solder paste 13 is applied to only a limited area near the opening of the through hole 14.
If the solder reflow process is performed after providing the solder ball 10 to the through hole, the melted solder paste 13 in the through hole 14 is absorbed by the solder ball 10 as shown in FIG. 6B, which results in a state in which solder is not present in the through hole 14. Thus, there is a problem in that the solder ball 10 cannot be appropriately provided to the substrate 2 having the through hole 14 whose diameter L2 is small even if the ball mounting method shown in FIG. 5 is used. Hereinafter, the state in which the electric connection cannot be achieved due to an air gap formed between the solder ball 10 (as an external connection terminal) and the electrode film 5 is referred to as an open fault.
As shown in FIG. 7, if the diameter L3 of the through hole 7 is reduced, the diameter L4 of an electrode pad 16 provided on a mounting board 15 on which the semiconductor device 1A is mounted becomes larger relative to the through hole 7 (L3&lt;L4). Additionally, a solder plating 17 is applied to the electrode pad 16 so as to facilitate the bonding of the solder ball 10 to the electrode pad 16.
When the electrode pad becomes relatively larger than the through hole 7 and when the solder ball 10 and the solder plating layer 17 are melted due to the heat applied during the mounting process, the melted solder ball 10 is attracted by the electrode pad 16. Accordingly, there is a problem in that an air gap is formed in the through hole 7 as shown in FIG. 8, which results in the open fault.
Further, in the conventional semiconductor device 1A, there is a problem in that a crack frequently occurs in the via part 9 as shown in FIG. 9 when the semiconductor device 1A is mounted onto the mounting board 15. It is considered that the crack 19 occurs due to the difference in the thermal expansions between the semiconductor chip 3 and the mounting board 15.
In a semiconductor device using a flexible printed circuitboard (FPC) or TAB tape substrate, it is common for a semiconductor chip to be fixed to the FPC or the TAB tape substrate by an adhesive. In the flip chip type semiconductor device in which the circuit forming surface of the semiconductor chip faces a substrate, an insulating adhesive is used to fix the semiconductor chip to the substrate. That is, the semiconductor chip is fixed to the substrate by applying the insulating adhesive to the tape substrate on which copper (Cu) patterns are formed and curing the insulating adhesive by heating after the semiconductor chip is placed on the tape substrate. In this case, the attachment of the semiconductor chip can be easily and positively performed by managing an amount of the adhesive applied between the semiconductor chip and the tape substrate.
The semiconductor chip is sealed by a sealing resin after being fixed by the adhesive. Since the tape substrate, the circuit patterns, the adhesive, the semiconductor chip and the sealing resin are made of different materials, the thermal expansion rates of these members are different from each other. In the above-mentioned structure of the semiconductor device, these members are made in contact with each other. Accordingly, there are stresses generated between these members due to the differences in the thermal expansion rates. Among these members, the wiring pattern has the smallest mechanical strength. Thus, if the stress due to heat is repeatedly generated in the wiring pattern, a fault such as breakage of the wiring pattern or a fracture of the external terminal may occur.
Consideration is given to a case in which the thermal expansion rate of the adhesive is in the range of 10 to 16 [ppm/.degree. C.] and that of the seal resin is in the range of 6 to 10 [ppm/.degree. C.], where ppm/.degree. C. represents 10.times.10.sup.-6 /.degree. C. These materials generally have a low glass transition temperature (Tg). Specifically, the glass transition temperature of the adhesive is in the range of 135.degree. C. to 145.degree. C. and the glass transition temperature of the seal resin is 130.degree. C. The material having a low glass transition temperature is disadvantageous with respect to prevention of breakage of the wiring pattern. However, since the glass transition temperatures of the above-mentioned materials approximate each other, the thermal stress between these members can be reduced to a certain extent.
It is known that, in normal material, when the temperature of the material exceeds the glass transition temperature, the thermal expansion rate increases to more than three times that when the temperature is below the glass transition temperature. Accordingly, when the higher temperature of a heat cycle of an evaluation test of a semiconductor device is set to a temperature exceeding the glass transition temperature, the thermal stress generated in each member is greatly increased. Thus, the time period to reach the generation of a fault such as the breakage of wire may be extremely reduced. However, since the glass transition temperatures of the members approximate each other, the stress generated in the interface of these members is small.
On the other hand, in a case in which the thermal expansion rate of the adhesive is in the range of 30 to 40 [ppm/.degree. C.] and that of the seal resin is in the range of 12 to 16 [ppm/.degree. C.], the glass transition rate of the seal resin is as high as 210.degree. C. When the glass transition temperature of the seal resin is high, the warping of the semiconductor device can be reduced. However, the difference in the thermal expansion between the adhesive and the seal resin is large, and the glass transition temperatures are also different to a considerable extent, therefore the stress generated in the interface between the adhesive and the seal resin is increased. As a result, the possibility of occurrence of a fault such as breakage of the wiring pattern or a fracture of the external terminal is increased.